About 10 percent of the district's 46,000 students attend dual-language programs in Spanish, Japanese, Mandarin Chinese, Russian, and Vietnamese. Seats are oversubscribed and assigned by lottery, which allowed researchers to run a randomized trial of about 1,600 students who started kindergarten in 2004-2010. Schools with dual-language-immersion programs had comparable class sizes and resources.

Researchers compared students who were randomly assigned to the immersion programs to those who had applied unsuccessfully for the lottery. Those who participated in the immersion programs scored significantly higher on the Oregon Assessment of Knowledge and Skills in reading; by the equivalent of seven months of learning by the end of 5th grade and by nine months of learning by the end of 8th grade. Students who were native English speakers got the same boost as English-learners. There was no benefit, but also no negative effect, on immersion students' scores in math or science.

In Portland's two-way immersion programs, students start out in kindergarten learning 90 percent of the time in their native language, and 10 percent of the time in the language they are learning. The proportion of time spent in the target language increases 10 percent each year, until students in grades 4 and up learned for half of their time in each language.

Among students who started out as English-language learners, those who attended dual-language programs were 3 percentage points less likely to still be an ELL by 6th grade than English-learners who were not in dual-language.

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